The invention relates to a high-pressure metal halide lamp comprising:
a sealed light-transmittent discharge vessel having opposite seals and enveloping a discharge space which has a gas filling comprising rare gas and metal halides; PA1 tungsten electrodes oppositely disposed in the discharge space; PA1 current lead-through conductors located in a respective seal of the discharge vessel and issuing from the discharge vessel; PA1 electrode rods connected to a respective one of said lead-through conductors and carrying a respective one of said electrodes.
Such a lamp is known from U.S. Pat. No. 5,424,609.
The known lamp has a ceramic discharge vessel, current lead-through conductors of e.g. niobium or tantalum, and a gas filling of rare gas, mercury and a mixture of metal iodides including rare earth metal iodides, being the iodides of the lanthanide's, scandium and yttrium, as the metal halides.
In ceramic discharge lamps the current lead-through conductors generally extend into the discharge space, thereby being exposed to attack by the metal halides. In the known lamp the inner ends of the current lead-through conductors are embedded in ceramic sealing material of the seals and a respective conductor which is said to be halide-resistant at least as its surface issues from the seals and connects the lead-through conductors with tungsten electrode rods. The conductors at least at their surface consist of tungsten, molybdenum, platinum, iridium, rhenium, rhodium, or an electrically conducting silicide, carbide or nitride.
It was found that the known lamp suffers from a decreasing luminous output due to a blackening of the discharge vessel which is caused by the deposition of tungsten originating from the electrodes and the electrode rods.
A single ended quartz glass metal halide lamp is known from EP-A 0.343.625 in which the gas filling consists of rare gas, mercury and a mixture of metal iodides and metal bromides. Both lead-through conductors are embedded next to one another in the one seal of the discharge vessel and the electrode rods extend next to one another into the discharge space. Due to the elevated temperature of the electrode rods during operation and their short mutual distance, in such a lamp the discharge arc may jump over from the electrodes to the electrode rods, thereby approaching the discharge vessel and causing it to become overheated. The jump over of the discharge arc, however, also causes the electrode rods to become even more heated, to evaporate locally and thereby to blacken the discharge vessel and to become broken themselves. Moreover, the short distance in the kind of lamp between the electrode rods and the portion of the discharge vessel which is heated to softening in making the seal during manufacturing the lamp, causes tungsten electrode rods to become oxidized, which results in a fast blackening of the discharge vessel during operation.
In the lamp of EP-A 0.343.625 oxidation of the electrode rods and a jump over of the discharge arc are obviated in that the electrode rods at least at their surface consist of rhenium or rhenium-tungsten alloy. These electrode rods project through a tungsten electrode coil at their ends inside the discharge space. Rhenium is less liable to become oxidized and has a lower heat conductivity, whereby a rhenium electrode rod would assume a lower temperature during operation. It was found, however, that the lamp has the severe disadvantage to suffer from a rapid blackening due to evaporation of rhenium and deposition of rhenium on the discharge vessel.
A similar single ended quartz glass lamp and a double ended quartz glass lamp are known from U.S. Pat. No. 5,510,675. These lamps have a gas filling of rare gas, mercury and a mixture of metal iodides and bromides. Their electrode rods have at their end inside the discharge space a wrap winding of tungsten wire and a fused spherically shaped tungsten electrode head. The purpose thereof is to eliminate flicker which is caused by migration of the discharge arc. The electrode rods may consist of rhenium in stead of tungsten. It was found that the lamp having rhenium electrode rods suffers from a rapid blackening due to evaporation of rhenium and deposition of rhenium on the discharge vessel. In the event the electrode rods consist of tungsten, blackening of the discharge vessel may occur as a result of evaporation of tungsten from the electrode rods and the electrodes, and deposition on the discharge vessel. Moreover in this event, the electrode rods may locally become thinner and thinner, resulting in the breakage of the rods at a relatively early moment.